Controlled release fertilizer can effectively increase nutrient utilization, save labor and reduce threat to environment, with extensive applications in horticultural plants and cash crops. In China, the production and technology of controlled release compound fertilizer is relatively lagged behind, and the research on high efficient and low-cast equipment is almost a blank, therefore the popularizing of controlled release fertilizer on market has been slow.
Presently in China, two main types of coating machines are used: drum type and fluidized bed type, in the former, as materials are extruded and pressed together, bonding is likely to occur, and the film can be broken when they are separated. In the coating cavity, materials are in an orderless moving status, resulting in uneven coating and waste of coating material. Furthermore, the materials in the system are not in a sealed condition, and the thermal efficiency is low, therefore organic solvent leakage is likely, and it is not easy to recover it. Fluidized bed coating machine is an advanced coating device, as the nebulized particles of coating liquid move in the same moving direction of materials at a high and homogeneous speed, there is no adhesion at the spray port, with a short distance between nebulized particles to material granule surface, making it easy for nebulized particles to spread over the granule surface, saving coating material, and all areas of the granule surface have the same chance to come into contact with nebulized particles, thus ensuring continuous, compacted and homogeneous coating.
Although the fluidized bed coating technology has been in practice for years and the coating principle is relatively mature, there are still many practical problems when it is used in scaled production of coated fertilizer, for example, as the current coated fertilizer is mainly coated with organic solvent, the fluidized beds now in use are not suitable to water-based coating. First, during coating in this type of device, the external air is heated via a steam heat exchanger and flows into the coating machine main tower and through the fluidized bed bottom from the bottom to the top as hot air, carrying away the moisture in the coating emulsion and finally exhausted by the ID fan. This open system wastes thermal energy, especially, the air inflow is quite high in large scale coating production, sometimes it is not possible to reach the required air inlet temperature. Second, there are many problems in the air inlet of this type of devices, especially, when some Chinese medicine or urea sensitive to humidity is coated, the uncontrolled humidity can produce substantial environmental effect on the water-based coating. Third, many coating materials, such as acrylic acid emulsion can easily solidify in the spray gun at high temperature, resulting in gun blocking and failure of coating. But low air inlet temperature will lead to incomplete drying, resulting in adhesion. Fourth, as coating involves many parameters, which are directly related to coating quality and output. It is difficult to meet the current technical requirements of coating process repeatability with manual control.